1. Field of the Invention
The present invention relates to a differential amplifier for amplifying and communicating an analogue signal and a test circuit having the differential amplifier installed therein.
2. Description of the Related Art
In general, a bipolar transistor or BiCMOS transistor is used for a differential amplifier. In recent years, there has been an increasing demand for a system-on-chip configuration and reduction of power consumption, in response to which the CMOS has been widely used. The differential amplifier of the CMOS configuration is expected to satisfy requirements such as low distortion and high-speed operation.
FIG. 14 shows an example of a conventional differential amplifier of the CMOS configuration capable of class A output. The differential amplifier has a two-stage configuration, wherein an output-stage amplifier circuit A2 is connected to a terminal for an output signal DiffOUT of an input-stage differential amplifier circuit A1. The differential amplifier A1 is comprised of a constant current source E1, an input transistor Tr1, wherein a positive-side input signal INP is applied to a gate, an input transistor Tr2, wherein a negative-side input signal INM is applied to a gate, and transistors Tr3 and Tr4 for load resistance. The amplifier circuit A2 is comprised of a constant current source E2, a transistor Tr5 for amplification, and a phase compensation capacity C1, and generates and outputs an output signal OUT.
In the differential amplifier, when a resistive load is connected to an output terminal of the amplifier circuit A2, a current flow with respect to the transistor Tr5 decreases, resulting in signal communication at a lower speed. As a result, responses of the differential amplifier tend to be oscillatory.
When the resistive load is connected, a differential amplifier capable of class AB output with a source follower circuit or source ground amplifier circuit additionally installed therein is generally used, an example of which is shown in FIG. 15. In a first source follower circuit A4, the output signal DiffOUT of the differential amplifier A1 is applied to a gate of a transistor Tr6 for amplification, wherein a source is connected to a constant current source E3. Further, in an amplifier circuit A3, an antipolar transistor Tr7 is connected to a transistor Tr8. To a gate of the upper-side transistor Tr7 is applied a positive-side output signal OUTP from the first source follower circuit A4. A drain connection point, at which drains of the transistors Tr7 and Tr8 are connected to each other, serves as an output terminal.
In the foregoing configuration, a signal is communicated to both the transistors Tr7 and Tr8. Therefore, one of the transistors responds poorly, a responsiveness of the other transistor can be maintained. As a result, the connection of the resistive load to an output terminal of the amplifier circuit A3 does not deteriorate the response of the differential amplifier.
In FIG. 16, a grounded-source amplifier circuit A5 is further added. In a circuit shown therein, providing that the amplifier circuit A3 has a push-pull configuration to thereby enlarge an amplitude of an output signal, a waveform distortion can be controlled.
However, the class-AB differential amplifier, because of its number of stages larger than that of the class A differential amplifier, is unsuitable for a high-speed operation. When the output-stage transistors Tr7 and Tr8 are increased in size in order to improve the capability of the differential amplifier, the output signal DiffOUT becomes an excessive load, thereby decreasing the operation speed. In the case of FIG. 16, the transistors Tr7 and Tr8 respectively have different drive signals, which easily leads to the deterioration of the waveform of the output signal.